Semiconductor memory device

ABSTRACT

A semiconductor memory device may be formed from a pair of transfer MOS transistors  1, 2  controlled by a word line  11  and a pair of data retaining flip-flop circuit formed from serially connected load elements  5, 6  and drive MOS transistors  3, 4.  In the semiconductor memory device, the transfer MOS transistors  1, 2  have a threshold voltage greater than a threshold voltage of the drive MOS transistors  3, 4.  The memory device may display an improved β ratio, and reduce the size of the drive MOS transistors to thereby reduce the cell area.

[0001] This application is a Divisional of U.S. application Ser. No.09/995,574, filed Nov. 29, 2001, which is hereby incorporated byreference in its entirety. Applicant hereby incorporates by referenceJapanese Application No. 2000-363207, filed Nov. 29, 2000, in itsentirety.

TECHNICAL FIELD

[0002] The present invention relates to semiconductor memory devices,including an SRAM (static random access memory) with an improved βratio.

[0003] 1. Related Art

[0004] A conventional SRAM often uses a CMOS cell that uses sixtransistors, as it has a large operation margin and a small dataretaining current.

[0005] In order to stabilize the operation of the SRAM, the β ratiobetween the drive MOS transistor and the transfer MOS transistor needsto be made large. For this purpose, these transistors are conventionallymade in different sizes on their designs to secure a sufficient β ratio.For example, as a measure to increase the β ratio, sizes (gatewidth/gate length) of transfer MOS transistor and drive MOS transistorare changed. More specifically, when gate width/gate length of atransfer MOS transistor are respectively 0.18/0.18, gate width/gatelength of a drive MOS transistor are respectively 0.22/0.12, to therebyincrease the β ratio.

[0006] It is noted that the aforementioned β is a parameter that isdefined by β=μ CoxWeff/Leff, where μ is carrier mobility, Cox is acapacitance of gate oxide film, Weff is an effective channel width, andLeff is an effective channel length. The β ratio is defined by (thecapability of a drive MOS transistor)/(the capability of a transfer MOStransistor).

SUMMARY

[0007] Embodiments relate to a semiconductor memory device including apair of transfer MOS transistors controlled by a word line and a pair ofdata retaining flip-flop circuits formed from serially connected loadelements and drive MOS transistors, wherein the transfer MOS transistorseach has a threshold voltage greater than a threshold voltage of each ofthe drive MOS transistors.

[0008] Embodiments also relate to a semiconductor memory deviceincluding a pair of transfer MOS transistors controlled by a word lineand a pair of data retaining flip-flop circuits formed from seriallyconnected load elements and drive MOS transistors, wherein a gateelectrode of each of the transfer MOS transistors has an impurityconcentration lower than an impurity concentration of a gate electrodeof each of the drive MOS transistors.

[0009] Embodiments also relate to a semiconductor device including anSRAM. The device includes a first transistor including a gate electrodeconnected to a word line, the first transistor also including a firstend connected to a first bit line. The device also includes a secondtransistor including a gate electrode connected to the word line, thesecond transistor also including a first end connected to a second bitline. The device also includes a third transistor including a gateelectrode, the third transistor including a first end connected to aground potential and a second end connected to a second end of the firsttransistor. The device also includes a fourth transistor including agate electrode, the fourth transistor including a first end connected tothe ground potential and a second end connected to a second end of thesecond transistor. The device also includes a fifth transistor includinga gate electrode, the fifth transistor including a first end connectedto a power supply and a second end connected to the second end of thefirst transistor. The device also includes a sixth transistor includinga gate electrode, the sixth transistor including a first end connectedto the power supply and a second end connected to the second end of thesecond transistor. The gate electrode of the third transistor and thegate electrode of the fifth transistor are each connected to the secondend of the second transistor. The gate electrode of the fourthtransistor and the gate electrode of the sixth transistor are eachconnected to the second end of the first transistor. In addition, thefirst and second transistors have a threshold voltage that is greaterthan a threshold voltage of the third and fourth transistors.

[0010] Embodiments also relate to a method for forming a semiconductordevice including forming a pair of transfer MOS transistors controlledby a word line, and forming a pair of data retaining flip-flop circuitsfrom serially connected load elements and drive MOS transistors. Themethod includes forming the transfer MOS transistors to have a thresholdvoltage greater than that of the drive MOS transistors.

[0011] Embodiments also relate to a method for forming a semiconductordevice including forming a pair of transfer MOS transistors controlledby a word line, and forming a pair of data retaining flip-flop circuitsfrom serially connected load elements and drive MOS transistors. Themethod includes forming a gate electrode of each of the transfer MOStransistors to have a lower impurity concentration than that of a gateelectrode of each of the drive MOS transistors.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Embodiments of the invention are described with reference to theaccompanying drawings which, for illustrative purposes, are schematicand not necessarily drawn to scale.

[0013]FIG. 1 shows a circuit diagram of a SRAM as an example of asemiconductor memory device in accordance with an embodiment of thepresent invention.

[0014]FIG. 2 shows a plan view of a semiconductor memory device inaccordance with an embodiment of the present invention.

DETAILED DESCRIPTION

[0015] In the measures in which the β ratio is increased by changing thegate width/gate lengths of transistors as described in the related artsection above, the gate width/gate length of drive MOS transistors maybecome larger in size than a desired process limit, because the gatewidth/gate length of transfer MOS transistors are determined by theprocess limit that is attributable the process and devicecharacteristics. As a result, the cell area of the SRAM becomes large.

[0016] Preferred embodiments of the present invention have been made inview of the problems described above, and it is an object of preferredembodiments of the present invention to provide a semiconductor memorydevice that can reduce the cell area by improving the β ratio andreducing the size of drive MOS transistors.

[0017] A semiconductor memory device in accordance with a preferredembodiment of the present invention comprises: a pair of transfer MOStransistors controlled by a word line and a pair of data retainingflip-flop circuits formed from serially connected load elements anddrive MOS transistors, wherein the transfer MOS transistor has athreshold voltage greater than a threshold voltage of the drive MOStransistor.

[0018] According to the above-described semiconductor memory device, thethreshold voltage of the transfer MOS transistor may be made greaterthan that of the drive MOS transistor, such that the β ratio, which is aratio between the capability of the drive MOS transistor and thecapability of the transfer MOS transistor, can be increased.Furthermore, since the size of the drive MOS transistor can be reduced,the cell area can be reduced.

[0019] Also, in the above-described semiconductor memory device, achannel region of the transfer MOS transistor may have an impurityconcentration higher than an impurity concentration of a channel regionof the drive MOS transistor.

[0020] Also, in the above-described semiconductor memory device, Ge(germanium) may be introduced in a gate electrode of the transfer MOStransistor. As a result, the work function of the transfer MOStransistor can be increased, whereby the threshold voltage of thetransfer MOS transistor can be made greater than the threshold voltageof the drive MOS transistor. Accordingly, the β ratio can be improved,and the size of the drive MOS transistor can be reduced to therebyreduce the cell area.

[0021] A semiconductor memory device such as that described above may bepertinent to a semiconductor memory device comprising a pair of transferMOS transistors controlled by a word line and a pair of data retainingflip-flop circuits formed from serially connected load elements anddrive MOS transistors, wherein a gate electrode of the transfer MOStransistor has an impurity concentration lower than an impurityconcentration of a gate electrode of the drive MOS transistor.

[0022] By a semiconductor memory device in accordance with preferredembodiments of the present invention, the impurity concentrations of thegate electrodes may be adjusted such that depletion is more prone tooccur in the gate electrode of the transfer MOS transistor compared tothe gate electrode of the drive MOS transistor. As a result, thecapacity between the gate electrode of the transfer MOS transistor andthe silicon substrate is reduced, and the capability of the transfer MOStransistor is lowered. Accordingly, the β ratio can be improved, and thecell area of the SRAM can be reduced.

[0023] Also, in a semiconductor memory device in accordance with anembodiment of the present invention, a channel region of the transferMOS transistor may have an impurity concentration that is generally thesame as that of a channel region of the drive MOS transistor. By this,the threshold voltage of the transfer MOS transistor can be increased,and therefore the capability of the transfer MOS transistor can belowered, and as a result, the β ratio can be improved.

[0024] Certain preferred embodiments of the present invention aredescribed below with reference to the accompanying drawings. FIG. 1shows a circuit diagram of a SRAM as an example of a semiconductormemory device.

[0025] The SRAM is a CMOS type cell using six transistors, which isformed from a pair of transfer MOS transistors 1 and 2 that arecontrolled by a word line 11, and a pair of data retaining flip-flopcircuits formed from serially connected load elements that aretransistors 5 and 6 and drive MOS transistors 3 and 4.

[0026] Namely, the gate electrodes of the respective transfer MOStransistors 1 and 2 are connected to the word line 11, and one end ofeach of the transfer MOS transistors 1 and 2 is connected to one of thebit lines 12. Also, the other end of the transfer MOS transistor 1 isconnected to one end of the drive MOS transistor 3, one end of thetransistor 5, the gate electrode of the drive MOS transistor 4 and thegate electrode of the transistor 6. The other end of the transfer MOStransistor 2 is connected to one end of the drive MOS transistor 4, oneend of the transistor 6, the gate electrode of the drive MOS transistor3 and the gate electrode of the transistor 5. Also, the other ends ofthe transistors 5 and 6 are connected to power supply Vcc, and the otherends of the drive MOS transistors 3 and 4 are connected to groundpotential, as illustrated in FIG. 1.

[0027] The SRAM that is a semiconductor memory device in accordance withthe first illustrated embodiment is a CMOS type cell using sixtransistors, which is equipped with the transfer MOS transistors 1 and 2having a threshold voltage greater than a threshold voltage of the driveMOS transistors 3 and 4. By this, the β ratio, which is a ratio betweenthe capability of the drive MOS transistor and the capability of thetransfer MOS transistor, can be increased.

[0028] It is noted that, in the example of a SRAM in which the powersupply voltage is 1.8 V-2.5 V, the threshold voltage of the transfer MOStransistors 1 and 2 may preferably be set at 0.75 V-0.95 V, and thethreshold voltage of the drive MOS transistors 3 and 4 may preferably beset, for example, at 0.6-0.8 V.

[0029] Next, a specific method to increase the threshold voltage of thetransfer MOS transistors 1 and 2 greater than the threshold voltage ofthe drive MOS transistors 3 and 4 is described.

[0030] The dose of impurity ion to be ion-implanted in the channelregions of the transfer MOS transistors 1 and 2 is made greater than thedose of impurity ion to be ion-implanted in the channel regions of thedrive MOS transistors 3 and 4. Since the threshold voltage of thetransfer MOS transistors 1 and 2 is made higher by using the methoddescribed above, the β ratio can be improved, and the size of the driveMOS transistors can be reduced to a desired process limitation, suchthat the cell area of the SRAM can be reduced.

[0031] It is noted that ion implantation conditions for the channelregions of the transfer MOS transistors 1 and 2 may be set as follows.For example, when the gate electrode is composed of polysilicon and thegate dielectric film is a silicon oxide film having a thickness of about4.5 nm, the ion implantation condition may preferably be set such that aP-type impurity concentration generally becomes about 4×10¹⁷ cm⁻³. Also,ion implantation conditions for the channel regions of the drive MOStransistors 3 and 4 may preferably be set such that the same generallybecomes about 3×10¹⁷ cm⁻³.

[0032]FIG. 2 shows a plan view of a semiconductor memory device inaccordance with a second illustrated embodiment of the presentinvention. The semiconductor memory device is a SRAM formed from a CMOScell using six transistors, which is formed from transfer MOStransistors 1 and 2, drive MOS transistors 3 and 4 and transistors 5 and6 as load elements.

[0033] In this SRAM, Ge is introduced in the gate electrode 11 of thetransfer MOS transistors 1 and 2. By this, the work function of thetransfer MOS transistors can be increased, and as a result, thethreshold voltage of the transfer MOS transistors can be made greaterthan the threshold voltage of the drive MOS transistors. As aconsequence, the β ratio, which is a ratio between the capability of thedrive MOS transistor and the capability of the transfer MOS transistor,can be increased, and the size of the drive MOS transistors can bereduced to a desired process limitation, such that the cell area of theSRAM can be reduced.

[0034] Next, a specific method to introduce Ge in the gate electrode 11of the transfer MOS transistors 1 and 2 is described.

[0035] A polysilicon film is deposited on the entire surface includingthe gate oxide film by a CVD (chemical vapor deposition) method, and aresist film is provided on the polysilicon film.

[0036] Then, Ge is ion-implanted in the polysilicon film, using theresist film as a mask. The ion implantation condition in this instancemay preferably be set as follows. For example, when the gate electrodeis composed of polysilicon having a thickness of 200 nm, an accelerationvoltage of 180 Kev, and a dose of 1×10¹⁶ cm⁻² to 1×10¹⁷ cm⁻² maypreferably be used. As a result, Ge is introduced in the polysiliconfilm in regions where the transfer MOS transistors 1 and 2 are to beformed, and the Ge concentration in these regions is about 20%-50% tothe silicon.

[0037] Then, the polysilicon film is patterned, whereby the gateelectrodes 11 and 14 for the transfer MOS transistors 1 and 2, the driveMOS transistors 3 and 4, and the transistors 5 and 6 are formed.

[0038] The SRAM that is a semiconductor memory device in accordance withthe second illustrated embodiment is a CMOS type cell using sixtransistors, which is equipped with the transfer MOS transistors 1 and 2having the gate electrode with an impurity concentration lower than theimpurity concentration of the gate electrodes of the drive MOStransistors 3 and 4. In this manner, the impurity concentrations of thegate electrodes may be adjusted by changing the amount of impuritiesthat are introduced in the gate electrodes, such that depletion is moreprone to occur in the gate electrode of the transfer MOS transistorscompared to the gate electrode of the drive MOS transistors. As aresult, the capacity between the gate electrode of the transfer MOStransistor and the silicon substrate is reduced, and the capability ofthe transfer MOS transistor is lowered. Accordingly, the β ratio can beimproved, and the size of the drive MOS transistor can be reduced to adesired process limitation, such that the cell area of the SRAM can bereduced.

[0039] It is noted that the impurity concentration for the gateelectrode of the transfer MOS transistor may preferably be about 1×10¹⁹cm⁻³ to 5×10¹⁹ cm⁻³. Also, the impurity concentration for the gateelectrode of the drive MOS transistor may preferably be about 1×10²⁰cm⁻³ to 6×10²⁰ cm⁻³.

[0040] In the case of this SRAM embodiment, the impurity concentrationof the channel region of the transfer MOS transistor may preferably bethe same as that of the channel region of the drive MOS transistor. As aresult, the threshold voltage of the transfer MOS transistor can beincreased, and therefore the capability of the transfer MOS transistorcan be lowered, such that the β ratio can be made greater.

[0041] It is noted that an ion implantation may be conducted one time tointroduce ions in the channel regions of the transfer MOS transistorsand the channel regions of the drive MOS transistors such that theimpurity concentrations in both of the channel regions can be madeapproximately the same with respect to one another.

[0042] It is noted that the present invention is not limited to theembodiments described above, and can be modified and implemented in avariety of manners.

[0043] As described above, in the preferred embodiments of the presentinvention, the threshold voltage of transfer MOS transistors is madegreater than that of drive MOS transistors. As a result, there can beprovided a semiconductor memory device that can reduce the cell area byimproving the β ratio, and reducing the size of the drive MOStransistor.

What is claimed:
 1. A method for forming a semiconductor devicecomprising: forming a pair of transfer MOS transistors controlled by aword line; and forming a pair of data retaining flip-flop circuits fromserially connected load elements and drive MOS transistors; wherein thetransfer MOS transistors are formed to have a threshold voltage greaterthan that of the drive MOS transistors.
 2. A method for forming asemiconductor device as in claim 1, comprising: implanting a first doseof an impurity ion into a channel region of each of the transfer MOStransistors; implanting a second dose of an impurity ion into a channelregion of each of the drive transistors; wherein the first dose isgreater than the second dose.
 3. A method for forming a semiconductordevice as in claim 1, further comprising introducing Ge into a gateelectrode of each of the transfer MOS transistors.
 4. A method forforming a semiconductor device comprising: forming a pair of transferMOS transistors controlled by a word line; forming a pair of dataretaining flip-flop circuits from serially connected load elements anddrive MOS transistors; and forming a gate electrode of each of thetransfer MOS transistors to have a lower impurity concentration thanthat of a gate electrode of each of the drive MOS transistors.
 5. Amethod for forming a semiconductor device as in claim 4, comprisingforming a channel region of each the transfer MOS transistors to have animpurity concentration that is the same as that of a channel region ofeach of the drive MOS transistors.
 6. A method for forming asemiconductor device including an SRAM, comprising: forming a firsttransistor to include a gate electrode connected to a word line and toinclude a first end connected to a first bit line; forming a secondtransistor to include a gate electrode connected to the word line and toinclude a first end connected to a second bit line; forming a thirdtransistor to include a gate electrode and to include a first endconnected to a ground potential and a second end connected to a secondend of the first transistor; forming a fourth transistor to include agate electrode and to include a first end connected to the groundpotential and a second end connected to a second end of the secondtransistor; forming a fifth transistor to include a gate electrode andto include a first end connected to a power supply and a second endconnected to the second end of the first transistor; forming a sixthtransistor to include a gate electrode and to include a first endconnected to the power supply and a second end connected to the secondend of the second transistor; forming the gate electrode of the thirdtransistor and the gate electrode of the fifth transistor to each beconnected to the second end of the second transistor; forming the gateelectrode of the fourth transistor and the gate electrode of the sixthtransistor to each be connected to the second end of the firsttransistor; and forming the first and second transistors to have athreshold voltage that is greater than a threshold voltage of the thirdand fourth transistors.
 7. A method for forming a semiconductor deviceas in claim 6, further comprising supplying a power supply voltage inthe range of 1.8 V to 2.5 V, a threshold voltage in the range of 0.75 Vto 0.95 V for the first and second transistors, and a threshold voltagein the range of 0.6 V to 0.8 V for the third and fourth transistors. 8.A method for forming a semiconductor device as in claim 6, furthercomprising: forming the first and second transistors to each include achannel region having an impurity ion implanted therein; forming thethird and fourth transistors to each include a channel region having animpurity implanted therein; and providing a dose of an impurity ionimplanted into the channel regions of the first and second transistorsthat is greater than a dose implanted into the channel regions of thethird and fourth transistors.
 9. A method for forming a semiconductordevice as in claim 8, further comprising: forming the gate electrode foreach of the first, second, third and fourth transistors to includepolysilicon and forming each of the first, second, third and fourthtransistors to include a gate dielectric layer comprising silicon oxidehaving a thickness of approximately 4.5 nm; and providing a P-typeimpurity concentration for the channel regions of the first and secondtransistors that is approximately 4×10¹⁷ cm⁻³, and a P-type impurityconcentration for the channel regions of the third and fourthtransistors that is approximately 3×10¹⁷ cm⁻³.
 10. A method for forminga semiconductor device as in claim 6, further comprising forming thefirst and second transistor gate electrodes to each comprise polysiliconand germanium.
 11. A method for forming a semiconductor device as inclaim 10, further comprising forming the first and second transistorgate electrodes to each comprise polysilicon with germanium implantedtherein, wherein the germanium concentration is 20 to 50 percent that ofthe silicon.
 12. A method for forming a semiconductor device as in claim6, further comprising forming the first and second transistor gateelectrodes to each include an impurity concentration that is less thanthat of the third and fourth transistor gate electrodes.
 13. A methodfor forming a semiconductor device as in claim 12, further comprisingforming the impurity concentration of the first and second transistorgate electrodes to be in the range of 1×10¹⁹ cm⁻³ to 5×10¹⁹ cm⁻³, andthe impurity concentration of the third and fourth transistor gateelectrodes to be in the range of 1×10²⁰ cm⁻³ to 6×10²⁰ cm⁻³.
 14. Amethod for forming a semiconductor device as in claim 12, furthercomprising: forming the first and second transistors to each include achannel region having an impurity ion implanted therein; forming thethird and fourth transistors to each include a channel region having animpurity implanted therein; and providing an impurity concentration inthe channel regions of the first and second transistors that isapproximately the same as an impurity concentration of the channelregions of the third and fourth transistors.